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Classification of Automotive Steel

Automotive

Steels

Low Strength

Steels

IF

steels

Mild

Steels

Conventional

HSS

C-Mn

Steels

Bake-Harde

nable

Steels

HSLAsteels

Advanced HSS

DPSteels

PFHTSteels

CPSteels

TRIPSteels

TWIPsteels

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High Strength Steels

Yield Strength from 210 to 550 Mpa

UTS from 270 to 700 Mpa

Ultra High Strength SteelsYield Strength greater than 550 Mpa

UTS greater than 750 Mpa

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Dual Phase Steels

Consist of a ferritic matrix

containing a hard martensitic

second phase in the form of

islands.

Produced by controlled cooling

from the austenite phase (in hot-

rolled products) or from the two-phase ferrite plus austenite phase (for continuously annealed

cold-rolled and hot-dip coated products) to transform some

austenite to ferrite before a rapid cooling transforms the

remaining austenite to martensite.

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The soft ferrite phase is generally continuous, giving these steels

excellent ductility. When these steels deform, strain is concentrated in

the lower-strength ferrite phase surrounding the islands of martensite,

creating the unique high work-hardening rate exhibited by these steels.

This Figure compares the engineering stress-strain curve for HSLA steel to a DP steel curve

of similar yield strength. The DP steel exhibits higher initial work hardening rate, higher

UTS, and lower YS/TS ratio than the similar yield strength HSLA.

Thus,the work hardening rate plus excellent elongation creates DP steels with much

higher UTS than conventional steels of similar yield strength.

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Other Advantages of DP steels

Carbon enables the formation of martensite at practical cooling rates by

increasing the hardenability of the steel.

Carbon also strengthens the martensite as a ferrite solute strengthener, as

do silicon and phosphorus.

DP steels also have a bake hardening effect which is the increase in yield

strength resulting from elevated temperature aging (created by the curing

temperature of paint bake ovens) after prestraining (generated by the

work hardening due to deformation during stamping or othermanufacturing process).

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Complex Phase steels

Very high ultimate tensile strengths.

The microstructure contains small amounts of martensite, retained austenite and

pearlite within the ferrite/bainite matrix.

Extreme grain refinement icreated by retarded recrystallization or precipitation of

microalloying elements like Ti or Cb.

Significantly higher yield strengths at equal tensile strengths of 800 MPa and

greater compared to DP steels.

Characterized by high energy absorption and high residual deformation capacity.

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TRIP steels

Microstructure is retained austenite

embedded in a primary matrix of ferrite. In

addition to a minimum of five volume percent

of retained austenite, hard phases such as

martensite and bainite are present in varying

amounts.

TRIP steels typically require the use of an

isothermal hold at an intermediate

temperature, which produces some bainite.

The higher silicon and carbon content of TRIP

steels also result in significant volume fractions

of retained austenite in the final structure.

TRIP steels use higher quantities of carbon than DP steels to obtain sufficient carbon

content for stabilizing the retained austenite phase to below ambient temperature.

Higher contents of silicon and/or aluminium accelerate the ferrite/bainite formation.

Suppressing the carbide precipitation during bainitic transformation appears to be

crucial for TRIP steels. Silicon and aluminium are used to avoid carbide precipitation inthe bainite region.

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Advantages of TRIP steel

Excellent formability,can be used in most severe stretch-forming

applications.

Exhibit high work hardening during crash deformation for excellent

crash energy absorption.

Very high UTS.

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During deformation, the dispersion of hard second phases in soft ferritecreates a high work hardening rate, as observed in the DP steels. However, inTRIP steels the retained austenite also progressively transforms to martensitewith increasing strain, thereby increasing the work hardening rate at higherstrain levels.

In this figure,engineering stress-strain behaviour of HSLA, DP and TRIP steelsof approximately similar yield strengths are compared. The TRIP steel has alower initial work hardening rate than the DP steel, but the hardening ratepersists at higher strains where work hardening of the DP begins to diminish.

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TWIP steels

High manganese content (17-24%) that causes the steel to be fully

austenitic at room temperatures.

A large amount of deformation is driven by the formation of deformation

twins.

The twinning causes a high value of the instantaneous hardening rate (n

value) as the microstructure becomes finer and finer. The resultant twinboundaries act like grain boundaries and strengthen the steel.

TWIP steels combine extremely high strength with extremely high

stretchability.

The n value increases to a value of 0.4 at an approximate engineering

strain of 30% and then remains constant until both uniform and totalelongation reach 50%.

The tensile strength is higher than 1000 Mpa.